Electrode modification using an unzippable polymer paste

ABSTRACT

A paste is described for capping electrodes with an oxide free metal layer incorporating a solvent, an unzippable polymer and particles. The electrode could be an interconnect such as a C4 bump. A method for forming a coating and for testing integrated circuit chips is also described. The invention overcomes the problem of interconnecting Pb containing electrodes that are covered with an insulating oxide on integrated circuit chips by coating the Pb containing electrode with Au to provide an oxide free surface for testing and interconnection.

FIELD OF THE INVENTION

This invention relates to a paste and more particularly to a pasteincluding an unzippable polymer, solvent and conductive particles forcoating electrodes such as C4 bumps with oxide free conductive particlesfor applications such as low temperature interconnections between anintegrated circuit chip and a substrate made of polymer/fillercomposites, such as an FR4 printed circuit board.

BACKGROUND OF THE INVENTION

With more and more transistors being placed on a chip to increase thenumber of functions, the number of input/output (I/O) pads perintegrated circuit chip has increased significantly. The increase in thenumber of I/O pads per chip are making traditional bonding methods, suchas wire bonding (WB) and tape automated bonding (TAB) difficult. Flipchip attach (FCA), which is usually an area array in contrast to aperipheral array for WB and TAB, is becoming increasingly pervasive dueto the number of pads. In FCA, the chip is bumped with a lead-rich Pb/Snalloy ball using metal deposition through a resist-mask, for example.The bonding of this chip is achieved by self alignment and placing thechip on the substrate which has been covered with high viscosity flux toreduce oxides. The chip is held in place by the flux. The whole assembly(chip and substrate) is subsequently heated in the range from 350 to400° C. to a temperature which melts the solder forming an interconnectbetween balls or bumps on a chip and respective pads on a substrate.

Conventionally the substrates were multi-layer ceramic (MLC) structuresthat could withstand temperatures up to 400° C. Dictated by both thenumber of pads and lower cost, there is a growing need to attach similarC4 bumped integrated circuit chips to organic substrates made ofpolymer/filler composites, such as FR4. Such organic substrates degradeat solder reflow temperatures above 300° C. Thus a low temperaturejoining material is needed to attach the C4 bumps of a chip torespective substrate pads.

One method to attach electrodes such as C4 bumped chips to an organicsubstrate is by capping the C4 bumps first with a low temperaturemelting Pb/Sn-eutectic solder such as described in U.S. Ser. No.08/710,992 filed Sep. 25, 1996 by Berger et al. entitled "Method forMaking Interconnect for Low Temperature Chip Attachment" (YO996073) andassigned to the assignee herein. The Pb/Sn solder cap over the C4 bumpmay be accomplished by vapor depositing the metal components through aresist mask, followed by a solder reflow step. The masking processrequires expensive alignment and lithographic steps, and the vapordeposition process is costly due to high vacuum processing. The bondingis accomplished by reflowing the Pb/Sn-eutectic solder at temperaturesbelow 250° C. using acidic flux. Subsequently, the flux is removed usingorganic solvents that may be chloro-fluoro-carbon (CFC) based.

SUMMARY OF THE INVENTION

In accordance with the present invention, a paste is describedcomprising a solvent for an unzippable polymer, an unzippable polymerdissolved in the solvent to form a solution, and particles suspended inthe solution. The particles may be suitable for coating the surface of aselected material; and the polymer may comprise 10 wt. % or greater ofthe solution.

The invention further provides a coating comprising a substrate and alayer of the paste mentioned above that leaves a layer of particleswhich may be heated to form an alloy with the substrate material.

The invention further provides a method for testing the integratedcircuit chip with C4 bumps that are coated using the disclosed method.The particles in the paste being conductive and adherent to the C4surface will provide a conductive path between the C4 bumps andconductive pads on the test probe.

The invention further provides a method for coating C4 electrodes orbumps on an integrated circuit chip comprising the steps of applying apaste mentioned above to the surface of the C4 electrodes, the particlesin the paste being conductive and adherent to the surface of thesubstrate, and heating the paste to remove the solvent and theunzippable polymer wherein the particles may alloy with the Pb in theC4.

The invention provides a low cost C4 capping method.

The invention provides a method that does not require any lithography,alignment or vacuum processing steps.

The invention provides a bonding process that uses conductive adhesiveon the substrate that does not require any flux, hence no cleaning step.The conductive adhesive may typically be a silver or gold filled epoxy.

The invention provides a bonding process using a conductive organiccomposite that allows for greater thermal mismatch between the chip andthe substrate than a solid metal solder joint would.

The invention provides a bonding process that allows flip chip to padson organic substrates where the mismatch in the thermal coefficient ofexpansion (TCE) is significantly larger than the TCE between anintegrated circuit chip and a ceramic substrate.

The invention provides a testing process that would allow the C4 bumpsto be tested by conventional methods of pressure contact before bonding.

BRIEF DESCRIPTION OF THE DRAWING

These and other features, objects, and advantages of the presentinvention will become apparent upon consideration of the followingdetailed description of the invention when read in conjunction with thedrawing in which:

FIG. 1A shows one embodiment of the invention.

FIG. 1B shows an intermediate step in using the embodiment of FIG. 1A.

FIG. 1C shows the final result of using the embodiment of FIG. 1A.

FIGS. 2-4 show a sequence of steps for placing a coating of unzippablepaste on C4 bumps of an integrated circuit.

FIG. 5 is a SEM image of a C4 bump after a heating cycle having a metalcoating as in FIG. 1C.

FIG. 6 is a graph of data from Energy Dispersive X-ray Analysis of agold coated site on the C4 bump after the coating process, and

FIG. 7 is a graph of data from Energy dispersive X-ray Analysis of anuncoated site of lead on the C4 bump in FIG. 6.

FIG. 8 shows the final structure of a C4 bump coated with oxide freeparticles to be bonded to a paste bumped substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1A, a substrate 10 and a coating of unzippable paste13 is shown. Unzippable paste 13 comprises an unzippable polymer 15and/or their blends dissolved in solvent 18, particles 17 such asoxide-free metal. An oxide-free metal or surface is one that does notform an oxide layer to permit a conductivity greater than 1 ohm-cm orthe oxide layer is conducting with a conductivity greater than 1 ohm-cm.For example, C4 bumps of an alloy of 97 wt. % Pb and 3 wt. % Sntypically have an oxide coating or layer which provides a contactresistance which is greater than 1 ohm-cm without reflow of the solderin the C4 bump. Unzippable polymer 15 is a polymer that completelyvolatilizes above a certain temperature defined as the unzippingtemperature, T_(unzip). Some examples of unzippable polymers 15 includepoly(methyl methacrylate) (PMMA), poly(∝-methyl styrene) (PAMS), poly(propylene carbonate), poly(ethylene carbonate), and poly(chloral). Allthese polymers have a T_(unzip) below 400° C.

Solvent 18 is one that may be organic and dissolves the above mentionedpolymers. For example, N-methyl pyrrolidinone (NMP) is a good solventfor PMMA and PAMS.

Particles 17 may be composed of one or more metals. At least one of themetal components should form an alloy with Pb or Sn wherein the alloymelts in the temperature range from 150 to 400° C. For example, particle17 may be Au, Sn, or Au/Sn alloy. The particle should be oxide free orwith an oxide that is conductive. The particles could also be made ofcomposite material where the inner core may be ceramic or metal and theouter layer is an oxide free metal or a conductive oxide. Particles inthe paste may be in the range from 1 to 50 volumetric %.

Solvent 18 in paste 13 is removed by drying at T_(dry) under a selectedenvironment, leaving polymer 15 and particles 17 on the surface ofsubstrate 10. The temperature is then raised above the unzippingtemperature T_(unzip) of unzippable polymer 15 which results in thedegradation and evaporation of the polymer components.

FIG. 1C shows the final structure of substrate 10 coated with particles17 after unzipping. The T_(dry) is more than 50° C. below the T_(unzip).The selected environment may be air, N₂, vacuum, forming gas, Ar, He ora combination thereof. Coating 12 may be patterned or uniform oversubstrate 10. Selective coating 12 may be achieved by a standarddispensing process such as screening, stenciling, etc. Particles 17shown in FIG. 1C may be in a monolayer or multilayer. Particles 17 mayform an interconnected percolating structure or discrete islands.

FIGS. 2-4 show steps for coating C4 bumps of an integrated circuit withunzippable paste 13. Unzippable paste 13 is screened on a smooth, cleansurface of a solid substrate 22 through an interposer 24. Substrate 22may be glass, silicon or ceramic. Interposer 24 may comprise polyimide,polyester or other organic material or metal such as molybdenum, nickel,stainless steel, etc. Interposer 24 may have a plurality of openings orvias 26 which are slightly larger than the diameter of the C4 bumps 27to be coated. The thickness of interposer 24 is less than the height ofthe C4 bump 27. Typically, the diameter of C4 bump 27 is 75 μm. Thediameter of opening 26 may be in the range from 85 to 100 μm and thethickness of interposer 24 may be 50 μm. FIG. 2 shows openings 26 filledwith unzippable paste 13. Interposer 24 can be made by chemical etch,laser ablation or other conventional means to match the pattern of C4bumps on a chip or wafer.

Some other method of transferring the unzippable paste 13 to C4 bumps 27may be by pin transfer, dipping, brushing, etc.

Substrate 22 is then placed over or under a chip or wafer 34 of undicedor embedded chips having a plurality of C4 bumps 27 thereon. Openings 26filled with unzippable paste 13 are aligned with corresponding bumps 27on wafer 34. The alignment is done by self aligning the chip with thevias 26 in interposer 24. Substrate 22 is brought closer to chip orwafer 34 so that C4 bumps 27 are pressed into openings 26 and in contactwith unzippable paste 13 as shown in FIG. 3. Substrate 22 is thenseparated from or moved away from chip or wafer 34 so that C4 bumps 27are separated from contact with unzippable paste 13 in openings 26. Athin layer 29 of unzippable paste 13 adheres to C4 bumps 27 as the chipor wafer 34 is removed from proximity of substrate 22 as shown in FIG.4. In this example, the geometry of each C4 is well defineddimensionally and by composition, the contact area to unzippable paste13 is nominally identical for each bump. As a result, the thickness andsize of the paste film forming thin layer 29 on the C4 bumps 27 arenominally of the same thickness. The thickness of thin layer 29 may bein the range from 0.1 to 50 μm with a thickness variation in the rangefrom 0.05 to 25 μm. The process can be repeated a number of times byheating to T_(dry) to increase the amount of particles adhering to theC4 bumps.

Thin layer 29 is first dried by heating to T_(dry), then heated aboveT_(unzip) and the melting temperature of the solder metals in the C4bumps 27. The particles embed into the molten C4 or react with soldermetal, to form a strong bond between the particle and the C4.

Depending upon the particular unzippable polymer, metal, alloy, or othermaterial selected. T_(unzip) may be above or below the meltingtemperature T_(melt) of the metal, alloy or other material. WhereT_(unzip) is above T_(melt) heating may be above T_(unzip) in one step.Where T_(unzip) is below T_(melt), heating may first be above T_(unzip)and below T_(melt) and then raised above T_(melt).

Best practice is to completely remove the unzippable polymer prior toraising the temperature to T_(melt) or above.

Heating is done typically in an oven or zone furnace with a nitrogen orpreferably forming gas environment. During this thermal cycle, theunzippable paste 13 unzips and vaporizes and the metal filler particles17 fuse with respective C4 bumps forming an oxide-free conductivecoating 32 on C4 bumps 27.

FIG. 5 shows an image of conductive coating 32 on a C4 bump 27 taken byScanning Electron Microscopy (SEM). The image in FIG. 5 was taken afterthe heating cycle was completed at 360° C. for 30 mins. in a forming gasenvironment. Conductive coating 32 as shown in FIG. 5 is a uniform layeror coating of particles 17 which are Au.

FIG. 6 is a graph of Energy Dispersive X-ray Analysis of a site onconductive coating 32 on the C4 bump 27 shown in FIG. 5. In FIG. 6 theordinate represents X-ray intensity and the abscissa represents energyin kev. Curve 42 shows the X-ray intensity as a function of energy from0 to 20 kev. Peak 44 shows that the top surface of conductive coating 32is Au with very small traces of Pb. Curve 44 illustrates that conductivecoating 32 has a high coverage of gold over C4 bump 27.

FIG. 7 is a graph of Energy Dispersive X-ray Analysis of a site on C4bump 27 without conductive coating 32. FIG. 7 is formed from dataobtained from the C4 bump 27 shown in FIG. 5. In FIG. 7, the ordinaterepresents X-ray intensity and the abscissa represents energy in kev.Curve 46 shows the X-ray intensity as a function of energy from 0 to 20keV. Peak 48 shows that the surface of C4 bump 27 is Pb.

A process for bonding chips 50 to a substrate 52 is now described. Thechips 50 to be bonded would be processed to form, as described above, alayer 32 of particles 17 on its C4 bumps 27. A conductive adhesive paste54 comprising a polymer binder and noble metal particles would be placedon electrical pads 55 on substrate 52. The polymer binder may be athermoplastic or thermoset polymer. The paste may also contain a solventsystem that dissolves the polymer binder. Chip 50, after C4 bumps 27 areprocessed to contain a Au layer 32, is bonded to paste 54 or paste bump56 on substrate 52 by a method shown in FIG. 8. An example of aconductive paste suitable for use herein is described in U.S. Pat. No.5,086,558 by Grube et al. which issued Feb. 11, 1992 and in U.S. Pat.No. 5,566,044 by R. Booth et al. filed Oct. 21, 1996, both of which areincorporated herein by reference. Typically if paste 54 is epoxy based,chip 50 is bonded by heating the assembly in a temperature range from150 to 250° C. with 0 to 100 psi pressure. If paste 54 is thermoplastic,the temperature range is the same but the pressure can range from 10 to100 psi. The attached chip 50 is then underfilled by an encapsulant toform a typical finished assembly.

A process of testing the chip is now described. The C4 bumps of the chipto be tested would be coated with layer 32 by a process described above.The C4 bumps are electrically connected to the pads of the test probe byeither physical contact using force or bonding using the thermoplasticpaste method described above. For both methods, the contact pads of thetest probe are oxide free.

C4 bumped chips are needed to be attached to FR4 or other organicsubstates. Such assemblies will be used in computers, office equipment,automobiles and trucks, control systems, cellular phones, etc.

While there has been described and illustrated a process for forming ametal coating on a structure such as capping C4 bumps with Au to providean oxide free surface on C4 bumps on an integrated circuit chip, it willbe apparent to those skilled in the art that modifications andvariations are possible without deviating from the broad scope of theinvention which shall be limited solely by the scope of the claimsappended hereto.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is:
 1. A paste for forming a conductive coatingon a C4 bump containing Pb and Sn, said conductive coating having aconductivity greater than 1 ohm-cm comprising:a solvent for anunzippable polymer, an unzippable polymer selected from the groupconsisting of poly(α-methyl styrene) (PAMS), poly (propylene carbonate),poly (ethylene carbonate) and poly (chloral) dissolved in said solventto form a solution, and particles selected from the group consisting ofAu, Sn and Au/Sn alloy suspended in said solution, said particlessuitable for coating the surface of a C4 bump containing Pb and Snwherein said particles form a first alloy with Pb or Sn wherein saidfirst alloy melts in the temperature range from 150° C. to 400° C., saidunzippable polymer comprising 10 wt % or greater of said solution. 2.The paste of claim 1 wherein said unzippable polymer vaporizescompletely above a predetermined temperature.
 3. The paste of claim 1wherein said unzippable polymer vaporizes completely in the range from250° C. to 450° C.
 4. The paste of claim 1 wherein said solvent is aliquid selected from the group consisting of ketones, alcohols, cyclicorganic compounds, and aliphatic organic compounds.
 5. The paste ofclaim 1 wherein said unzippable polymer has a T_(unzip) less than 400°C.